1. Technical Field
This disclosure relates to a martensitic stainless steel sheet having superior corrosion resistance, toughness at weld zones and workability, and to a method for making the same. In particular, the disclosure relates to a martensitic stainless steel sheet for use in structural components of railway vehicles, automotives, buses, and the like formed by bending and to a method for making the same.
2. Description of the Related Art
Structural components of vehicles, namely railway vehicles, must have high corrosion resistance to maintain cosmetic appearance and to prevent a decrease in strength resulting from thickness reduction due to corrosion. Accordingly, austenitic stainless steel sheets, such as SUS301L and SUS304, having high corrosion resistance have been used in these structural components. Since hot rolled and annealed sheets or cold rolled and annealed sheets of austenitic stainless steel have poor strength, they are temper-rolled, utilizing strain induced martensitic transformation, to increase strength.
However, when vehicle structural components manufactured from austenitic stainless sheets are welded, the weld zones, where heat is input during welding, soften because the strains introduced during temper rolling become released, resulting in a decrease in strength and deterioration of favorable fatigue characteristics at the weld zones. In ferritic stainless sheets, grains in the weld zones coarsen and the toughness of the weld zones dramatically decreases, which is a problem. To overcome these problems, proposals to apply martensitic stainless steel sheets that do not suffer from softening of the weld zones and that have high toughness at the weld zones to vehicle structural components have been made.
For example, Japanese Unexamined Patent Publication No. 7-14542 teaches a martensitic stainless steel sheet having high strength, superior weldability, and high toughness.
However, the technology disclosed in Japanese Unexamined Patent Publication No. 7-14542 is directed to increasing the strength of the steel sheet, i.e., obtaining a high-toughness high-rust-resistance stainless sheet having a strength of 900 MPa or more. Hence, the steel sheet contains large amounts of Mn, Ni, Mo, N, and the like. When this steel sheet is bent, the outer portion of the bent portion cracks and, thus, this steel sheet is not suited for use in vehicle structural components such as those of railway vehicles, automotives, buses and the like, which is a problem.
Although technologies directed to obtaining martensitic stainless sheets having good corrosion resistance, toughness at the weld zones, and strength have been developed, no technology directed to martensitic stainless sheets suitable for use in structural components of vehicles, i.e., martensitic stainless sheets having high workability, particularly, high bendability, in addition to high corrosion resistance and toughness at the weld zones has been developed.